Chiang Mai Journal of Science

     The present study aims to identify effective fungicides and their inhibitory activities against the pathogenic fungi responsible for leaf spot disease in Hosta ventricosa: Fusarium oxysporum and F. ipomoeae. The inhibitory effects of five widely used fungicides were evaluated using the mycelial growth rate method. Furthermore, by focusing on factors such as spore germination, extracellular polysaccharide (EPS) content, superoxide dismutase (SOD) and peroxidase (POD) activities, and the morphology and structure of the mycelium, the study examined the inhibitory activity of the fungicides exhibiting superior control efficacy. The EC₅₀ values, which denote the concentrations required for 50% inhibition, were determined to be 0.03 and 0.21 μg/mL for F. oxysporum treated with prochloraz and 0.43 and 0.18 μg/mL for F. ipomoeae treated with pyraclostrobin. These findings indicate that both prochloraz and pyraclostrobin exhibited significant inhibitory effects. When prochloraz and pyraclostrobin were administered at a dose of 10×EC₅₀, significant increases in mycelial cell membrane permeability were observed, whilst spore germination rates decreased relative to the control and EPS generation was reduced. Both fungicides significantly modulated SOD and POD activities in a concentration-dependent and strain-specific manner. The results of the transmission electron microscopy and scanning electron microscopy observations demonstrated that the cell walls of the mycelium treated with pesticides exhibited signs of damage, characterised by a reduction and unevenness of the cytoplasmic content. In addition to these observations, notable changes in the morphology and structure of the intracellular cavity were identified. These findings imply that pyraclostrobin and prochloraz have potent antifungal properties, mainly affecting the mycelial cell membrane and increasing its permeability. This study contributes to the existing body of knowledge on the antifungal properties of pyraclostrobin and prochloraz, thereby providing a scientific foundation for the management of leaf spot disease in Hosta ventricosa.